Freelance
Traveller

This article originally appeared in the
August 2012 issue of
Freelance Traveller.

Mongoose Traveller uses a fairly
arbitrary and relative system to determine where planets are in a
system, relative to each other and the system’s sun. This works fairly
well when you use the standard gravitic drives and are prepared to ‘hand
wave’ away the exact distances travelled – giving a rough idea of how
long it will take to get from point A to point B is enough. However,
when you use reaction drives in the Babylon 5
setting, it can become critical – making sure you have enough fuel and
time to get from point A, through a Jump Gate, and to point B in another
system can be tricky to work out when you have no other points of
reference. Hopefully, this article will help to sort out a few of those
points of reference.

Jump Gate Positions

Jump Gates are large structures consisting of three, four (most
common), or five, ten-km-long vortex arrays arranged around a central
point where they generate a hole or gate connecting normal space with
hyperspace. The vortex arrays are not connected to each other, so rely
on small station-keeping thrusters to maintain their alignment. Because
of this, they are always found at Lagrange points oriented on a single
planet within a system, where they can maintain their position and
orientation with the minimum of effort. Similarly, because of their
size, they need a Lagrange point that is large enough to accommodate the
whole structure – this is why they are never constructed at Lagrange
points oriented on small planets or moons.

Lagrange
points are five points around a planet or moon where the gravitational
forces of the sun and other planets in that system roughly cancel out,
meaning that anything in those positions will orbit around the sun but
will stay (more or less) in the same spot, without being pulled into the
sun or another planet. They are normally identified as “L1” through
“L5”. The relative locations of the Lagrange points are shown in the
diagram to the right.

All of the jump gates in the explored galaxy fall into two main
types: those built by the original gate-builders, which are between
6,000 and 10,000 years old; and those built by the younger races, which
are less than 1,000 years old (about when the Minbari discovered
hyperspace travel and the gate system). The original, older gates are
found in many systems and are located at any of the Lagrange points, but
all usually around a planet in the outer zone – leading many scholars to
believe that they were built primarily as a means for the builders to
explore the galaxy.

Of the gates built by the younger races, those in the race’s home
world system are often located in the outer zone for security reasons,
as it gives any defences several days to prepare for an invasion
(indeed, when Earth finally gained jump gate technology it shut down the
Centauri-built gate in Earth orbit and built a new one near Jupiter).
Gates built by the younger races in colony systems are often built
around one of the planets in the habitable zone, where ships going to
and from the inhabited planets will have less distance to travel.

L3 jump gates are rare and are usually older gates built in
uninhabited systems. The younger races prefer not to build gates at this
point as they are too far away from any inhabited planets and, if in the
same orbit as an inhabited world, have the star between the planet and
the gate – from a security point of view, this gives hostile ships a big
advantage.

L4/L5 jump gates are quite common, and are usually older gates,
although due to the nature of these points, occasional asteroids or dust
clouds are found nearby. Many of the younger races prefer this position
when building a gate in a colony system as it is reasonably close to the
planet, but far enough away to give warning if hostile ships come
through.

L1/L2 points are fairly common in inhabited colony systems as they
provide a fast transit from the jump gate to the planet, which maximises
the potential for trade in the region.

Most planets will have at least one, if not several, space stations
in orbit around the nearest planet to the jump gate – as transfer points
between ships coming from hyperspace and ships coming up from the planet
or other planets in the system. Some planets even have space stations in
retrograde orbits (i.e., opposite to the normal rotation of the planet)
so that the station remains facing the jump gate at all times (such as
the Babylon 5 station). These space stations can be quite exotic, being
a meeting (and trading) place for the local race and aliens from all
over the explored galaxy.

Planet Locations

First, use the rules on pg 92-93 of Book 3:
Scouts to determine the numbers and positions of your planets in
a system. Once you have done that, work out the distances of the orbits
from the systems sun – to do that, use the (extremely arbitrary) system
here:

Inner Zone planets all have orbits roughly 0.25 AU apart (so the
first planet would be 0.25 AU from the sun, the second 0.5 and so on).

Habitable Zone planets all have orbits roughly 0.5 AU away from the
Inner Zone planets and each other (if there aren’t any Inner Zone
planets in your system, assume that the first planet orbits 0.5 AU from
the sun and work out from there).

Outer Zone planets start 1d3 AU from the Habitable Zone planets, and
then double each orbital distance after that.

Example: having worked out a system on the Scouts system, we end up
with 9 planetary orbits: 2 Inner Zone planets, 2 Habitable Zone planets
and 5 Outer Zone planets:

For simplification, I am rounding the Astronomical Unit (AU)
measurement to 150 million km.

Determine Jump Gate Location

Once you have worked out the planets and their orbits it’s time to
find out where the jump gate actually is and how far away it is. Roll
1d6 on the Focal Planet table to find out which planet the gate is
oriented on:

Focal Planet

Roll

Type of Planet

1

Inner Zone

2

Non-Mainworld Habitable Zone

3

Mainworld

4

Mainworld

5

Non-Mainworld Habitable Zone

6 or more

Outer Zone

DM +1 if major system; DM +2 if racial Home
system (not cumulative)

Having done this, roll another d6 to find out in what position the
gate is in:

Gate Location

Roll

Lagrange Point

1

Roll 1d6: 1-3, L1& 4-6, L2

2-3

L4

4-5

L5

6

Roll 1d6: 1-3, Choose; 4-5: L3; 6: Choose, and place a
second gate on a different focal planet.

From this, you can work out the distances from the gate to the planet
that it is oriented on:

Gate Distance

Lag. Pt.

Distance

L1, L2

Orbit Number/2 × 1 million km

L3

2 × Orbital Radius in millions of km

L4, L5

Orbital Radius in millions of km

The distinction between L1/L2 or L4/L5 is only important if the GM
wishes to use that info to flesh out additional details.

For our example system, L1/L2 are 1.5 million km, L3 is 300 million
km, and L4/L5 are 150 million km from the world.